What is a Primary treatment

Primary treatment is the initial phase in the process of treating wastewater at a dedicated treatment facility. Its main purpose is to remove large solids, floating materials and settleable organic and inorganic matter from sewage before it undergoes more advanced purification stages. This treatment plays a critical role in reducing the physical load on downstream processes and ensuring that the overall system operates efficiently.

Primary treatment focuses on the physical separation of contaminants, relying mainly on sedimentation and screening rather than biological or chemical processes. It typically removes between 50 to 70 percent of suspended solids and about 25 to 40 percent of the biochemical oxygen demand (BOD) from the incoming flow.

Purpose of primary treatment

The primary objectives of this stage are:

  • To remove gross solids such as grit, rags, plastic and paper from raw sewage

  • To reduce the organic load on secondary treatment systems

  • To protect downstream equipment from clogging, abrasion and fouling

  • To improve the efficiency of secondary biological processes

  • To facilitate the stable operation of the treatment plant

By separating out solids and settleable material early, the treatment plant reduces the risk of operational issues and ensures that the later stages can focus on removing dissolved and microscopic contaminants.

Key components of a primary treatment system

A typical primary treatment process involves several steps, all aimed at separating materials based on size, density and settleability:

1. Screening

The first line of defence in a wastewater facility involves bar screens or rotating drum screens that remove large objects such as:

  • Rags

  • Plastic bags

  • Sticks

  • Food wrappers

  • Sanitary products

These items are mechanically raked or filtered out and sent to landfill or incineration.

2. Grit removal

Grit chambers allow the removal of heavier inorganic particles such as:

  • Sand

  • Gravel

  • Broken glass

  • Metal fragments

These substances, while small, can cause significant wear and tear on pumps and pipelines if not removed early.

3. Sedimentation (primary settling tanks)

The heart of primary treatment is the primary clarifier or sedimentation tank. Wastewater flows slowly through large tanks, where:

  • Suspended solids settle by gravity to the bottom as primary sludge

  • Floating substances like grease, oil and scum rise to the surface

Sludge is collected from the bottom and skimmed materials are scraped from the top. Both are removed for further treatment or disposal.

Performance of primary treatment

Although not designed to remove dissolved pollutants, primary treatment significantly reduces:

  • Total suspended solids (TSS)

  • Biochemical oxygen demand (BOD)

  • Grease, oil and floating debris

  • Pathogens bound to solids

The level of treatment achieved depends on factors such as tank design, retention time and the characteristics of the incoming wastewater. Primary treatment typically achieves:

  • 50 to 70 percent removal of TSS

  • 25 to 40 percent removal of BOD

  • 60 to 90 percent removal of floatable oils and greases

These reductions make the wastewater suitable for biological secondary treatment or further filtration stages.

Integration with secondary and tertiary treatment

Primary treatment is not intended to produce water safe for discharge or reuse on its own. Instead, it prepares the wastewater for more advanced processes:

  • Secondary treatment involves biological methods such as activated sludge or trickling filters to remove dissolved and colloidal organic matter.

  • Tertiary treatment may include filtration, nutrient removal, disinfection or advanced oxidation to meet high-quality discharge or reuse standards.

Without effective primary treatment, these later stages would be overloaded and less efficient.

Sludge management

A significant by-product of primary treatment is primary sludge, which consists of organic and inorganic solids settled out during sedimentation. Sludge is typically:

  • Thickened to reduce its volume

  • Stabilised by anaerobic or aerobic digestion

  • Dewatered using presses or centrifuges

  • Disposed of by land application, incineration or landfill

Proper sludge handling is essential to prevent odours, pathogens and environmental contamination.

Benefits of primary treatment

Primary treatment offers several advantages:

  • Reduces the load on biological and chemical treatment processes

  • Protects mechanical equipment from blockages and abrasion

  • Minimises energy consumption in secondary treatment

  • Reduces the volume of pollutants entering the environment

  • Serves as a relatively low-cost and simple method of pollutant removal

In many small or rural facilities, primary treatment alone may be used temporarily before full systems are developed.

Limitations of primary treatment

Despite its benefits, primary treatment has limitations:

  • It does not remove dissolved substances, nutrients or many pathogens

  • Effluent still contains a significant pollution load

  • Requires additional treatment before water can be reused or safely discharged

Therefore, while essential, it is only the first step in comprehensive wastewater management.

Conclusion

Primary treatment is the foundational stage of wastewater processing, where gross solids, floatables and settleable materials are removed to protect equipment and improve the performance of downstream treatment processes. Though limited in the scope of what it can remove, its role is crucial in maintaining the overall efficiency, safety and environmental compliance of sewage treatment plants.

By effectively separating solids and reducing organic loads early in the process, primary treatment forms the backbone of all modern wastewater treatment strategies.